In such installations, in order to obtain high quality printing, it is necessary for the linear speed of the transfer system to be equal to the linear speed of the plate that is mounted on the plate-carrier cylinder so as to obtain contact without slip between the sheet and the plate during printing. The same applies to the peripheral linear speed of the backing system when constituted by a backing cylinder. In addition, in order to ensure uniform inking of the plate on each rotation of the plate-carrying cylinder, it is also necessary for the peripheral linear speed of the screened cylinder to be equal to the linear speed of the plate. In presently known installations, the various components mentioned above are coupled together mechanically and they are driven by a single motor. Consequently, the above-specified requirement for equal speeds is achieved for one given thickness of plate only. In practice, such installations are delivered by manufacturers with all of their components designed and organized to operate with a specific type of plate having a given thickness. Plate-carrier cylinders are machined as a function of the plate thickness specified by the printer so as to have a diameter, as measured on the plate, which is constant for a given type of machine.
Unfortunately, printers sometimes need to install on their installations plates of thicknesses other than that specified initially. Under such circumstances, print quality is degraded. When the thickness of the plate differs excessively from the thickness for which the installation was designed, the printer is constrained to work with a different installation designed to operate with the chosen plate thickness.
There also exist rotary presses for flexographic printing on a strip of continuous material, such presses including, in particular, a plate-carrier cylinder and a main cylinder round which the strip of material that is to be printed passes, said main cylinder enabling the material to be advanced and also acting as a backing cylinder. A rotary press of that particular type is already known, in particular from document FR 2 553 032, enabling the thickness of the plate used to be taken into account. That rotary press is fitted with at least two motors that are mechanically independent of each other, the first enabling the main cylinder to be rotated and the second enabling the plate carrier cylinder to be rotated. The position of the plate-carrier cylinder motor is servo-controlled relative to the main cylinder which is used as a reference, in such a manner that during the time in which the plate is in contact with the strip of material, the speed of rotation of the plate-carrier cylinder is such that the linear speed of the plate is equal to the peripheral linear speed of the main cylinder. On such machines, the developed size of the plate-carrier determines the printing pitch. The pitch can be varied a little. To do this, it is necessary to vary the speed of rotation of the plate-carrier cylinder during the time interval that the plate is not in contact with the material. This change in the speed of rotation of the plate-carrier cylinder during each 360.degree. revolution of the cylinder is possible only with plate-carrier cylinders of small inertia. In addition, the longer the plate mounted on the periphery of the plate-carrier cylinder, the greater the speed variation that needs to be imparted to the plate-carrier cylinder in order to conserve the distance between two printing patterns.
Unlike rotary presses of the kind described in document FR 2 553 032, sheet-fed printing machines have plate-carriers that are not interchangeable. Their dimensions are large and determine the maximum format of sheet that can be printed. They therefore have considerable inertia and the plates may cover nearly all of the developed surface of the plate-carrier cylinder.